Power converting apparatus, control device, and method for controlling power converting apparatus

ABSTRACT

A power converting apparatus is provided. The power converting apparatus includes a power converter configured to output a voltage to a load, and a controller configured to output a PWM signal which is generated in response to a voltage command to the power converter. The power converter includes a plurality of switching elements driven based on the PWM signal. The controller is configured to generate the PWM signal such that a first period during which a zero voltage is outputted and a second period during which a non-zero voltage is outputted are adjusted according to the voltage command. The controller is allowed to output the PWM signal which is set such that one first period and one or more second periods exist within an updating cycle of the voltage command, to the power converter.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure contains subject matter related to that disclosedin Japanese Priority Patent Application No. 2014-093048 filed JapanPatent Office on April 28, 2014. The contents of this application areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments disclosed herein relate to a power converting apparatus, acontrol device, and a method for controlling the power convertingapparatus.

Description of the Related Art

In a conventional power converting apparatus such as an inverter or thelike, there is known a technique in which a PWM (Pulse Width Modulation)signal is generated by comparing a carrier signal with a voltage commandand a switching element is controlled by the PWM signal.

In this power converting apparatus, there is known a technique in whicha switching loss is reduced by reducing a carrier frequency andperforming PWM control. For example, Japanese Patent ApplicationPublication No. 2011-109739 discloses a technique in which a switchingloss is reduced by switching a high carrier frequency and a low carrierfrequency depending on the magnitude of distortion of an output voltage.

SUMMARY OF THE INVENTION

In accordance with an aspect of an embodiment, there is provided a powerconverter converting apparatus for converting electric power between apower source and a load, including: a power converter configured tooutput a voltage to the load; and a controller configured to output aPWM signal which is generated in response to a voltage command to thepower converter, wherein the power converter includes a plurality ofswitching elements driven based on the PWM signal, wherein thecontroller is configured to generate the PWM signal such that a firstperiod during which a zero voltage is outputted and a second periodduring which a non-zero voltage is outputted are adjusted according tothe voltage command, and wherein the controller is allowed to output thePWM signal which is set such that one first period and one or moresecond periods exist within an updating cycle of the voltage command, tothe power converter.

In accordance with another aspect of the embodiment, there is provided acontrol device for controlling a power converter, including: a commandgenerator configured to generate a voltage command; and a signalgenerator configured to generate a PWM signal such that a first periodduring which a zero voltage is outputted and a second period duringwhich a non-zero voltage is outputted are adjusted according to thevoltage command, and output the PWM signal to the power converter,wherein the signal generator is allowed to output the PWM signal whichis set such that one first period and one or more second periods existwithin an updating cycle of the voltage command, to the power converter.

In accordance with still another aspect of the embodiment, there isprovided a method for controlling a power converting apparatus,including: a command generating process for generating a voltagecommand; and a signal generating process for generating a PWM signalsuch that a first period during which a zero voltage is outputted and asecond period during which a non-zero voltage is outputted are adjustedaccording to the voltage command, and outputting the PWM signal to apower converter, wherein the signal generating process includesoutputting the PWM signal which is set such that one first period andone or more second periods exist within an updating cycle of the voltagecommand.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a view showing a configuration example of a power convertingapparatus relating to a first embodiment.

FIG. 1B is an explanatory view of a first mode of a PWM control mode inthe power converting apparatus relating to the first embodiment.

FIG. 1C is an explanatory view of a second mode of the PWM control modein the power converting apparatus relating to the first embodiment.

FIG. 2 is a view showing a configuration example of the power convertingapparatus relating to the first embodiment.

FIG. 3A is an explanatory view of a first mode.

FIG. 3B is an explanatory view of a second mode in case where a voltagecommand is positive.

FIG. 3C is an explanatory view of a third mode in case where an outputvoltage command is positive.

FIG. 4 is a view showing a configuration example of a comparator shownin FIG. 2.

FIG. 5 is a flowchart illustrating one example of a processing flow of acontroller shown in FIG. 2.

FIG. 6 is a view showing a configuration example of a power convertingapparatus relating to a second embodiment.

FIG. 7 is a view showing a configuration example of a single-phase powerconverting cell.

FIG. 8 is a view showing a configuration example of a power convertingapparatus relating to a third embodiment.

FIG. 9A is an explanatory view of a first mode.

FIG. 9B is an explanatory view showing one example of the first mode.

FIG. 9C is an explanatory view showing another example of the firstmode.

FIG. 10 is a view showing a configuration example of a power convertingapparatus relating to a fourth embodiment.

FIG. 11A is an explanatory view of a first mode.

FIG. 11B is an explanatory view of the first mode.

FIG. 12 is a view showing a configuration example of another powerconvertor relating to the fourth embodiment.

FIG. 13 is a view showing a configuration example of a power convertingapparatus relating to a fifth embodiment.

FIG. 14 is an explanatory view of a space vector.

FIG. 15A is a view showing a relationship between a voltage vector, anoutput period and a PWM signal in a first mode.

FIG. 15B is a view showing a relationship between a voltage vector, anoutput period and a PWM signal in a second mode.

FIG. 15C is a view showing a relationship between a voltage vector, anoutput period and a PWM signal in the second mode.

FIG. 16 is a flowchart illustrating one example of a processing flow ofa controller shown in FIG. 13.

FIG. 17 is a view showing a configuration example of a power convertingapparatus relating to a sixth embodiment.

FIG. 18 is an explanatory view of a space vector method.

FIG. 19 is a view showing a correspondence example of a voltage commandvector and space vectors.

FIG. 20A is a view showing a relationship between a voltage vector, anoutput period and a PWM signal in a first mode.

FIG. 20B is a view showing a relationship between a voltage vector, anoutput period and a PWM signal in a second mode.

FIG. 21 is a view showing a configuration example of a power convertingapparatus relating to a seventh embodiment.

FIG. 22 is an explanatory view of a method for calculating the inversiontime of each of PWM signals.

FIG. 23 is a view showing a configuration example of a power convertingapparatus relating to an eighth embodiment.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of a power converting apparatus, a control device, and amethod for controlling the power converting apparatus disclosed hereinwill now be described in detail with reference to the accompanyingdrawings. It is noted that descriptions regarding a controller of thepower converting apparatus relating to each embodiment disclosed hereinalso serve as descriptions regarding an example of a control devicerelating to that embodiment. Further, it is noted that descriptionsregarding operations and a processing flow of the controller and itselements relating to each embodiment disclosed herein also serve asdescriptions regarding a method for controlling the power convertingapparatus relating to that embodiment. The present disclosure is notlimited to the embodiments to be described below.

[1. First Embodiment]

FIG. 1A is a view showing a configuration example of a power convertingapparatus relating to a first embodiment. As shown in FIG. 1A, the powerconverting apparatus 1 relating to the first embodiment convertselectric power supplied from a power source 2 to specified electricpower and outputs the specified electric power to a load 3. For example,if the power source 2 is a DC power source and if the load 3 is an ACmotor, the power converting apparatus 1 converts DC power supplied fromthe power source 2 to AC power and outputs the AC power to the load 3.The power source 2 may be, e.g., an AC power source and the load 3 maybe, e.g., a power system.

[1.1. Power Converting Apparatus 1]

The power converting apparatus 1 may include a power converter 10 foroutputting a voltage to the load 3 and a controller 20 for outputting aPWM signal which is generated in response to a voltage command to thepower converter 10.

The power converter 10 may include a plurality of switching elementsdriven based on a PWM signal (e.g., the PWM signal outputted from thecontroller 20) and may be connected between the power source 2 and theload 3. The power converter 10 outputs, e.g., an AC voltage power havinga single phase or multiple phases, to the load 3 via output lines 5 aand 5 b provided between the power converter 10 and the load 3.

The controller 20 generates the PWM signal such that a first periodduring which a zero voltage is outputted and a second period duringwhich a non-zero voltage is outputted are adjusted according to avoltage command. Further, the controller 20 is allowed to output to thepower converter 10 a PWM signal which is set such that one first periodand one or more second periods exist within an updating cycle of avoltage command. For instance, the controller 20 outputs, for eachupdating cycle of the voltage command, the PWM signal which causes onefirst period and one or more second periods combined within said eachupdating cycle of the voltage command.

The controller 20 may include a command generator 21 and a PWM signalgenerator 22. The command generator 21 generates a voltage command andoutputs the voltage command to the PWM signal generator 22. The voltagecommand is a signal whose voltage value or the like is referred to ingenerating a PWM signal. For example, the voltage command disclosedherein may be also regarded as a reference voltage and include one ormore phase voltage commands respectively corresponding to one or morephases of an AC voltage outputted from the power converter 10. However,for the sake of convenience, the voltage command relating to the presentembodiment will be described based on a case of a single phase ACvoltage. The command generator 21 may maintain or change a voltage valueof a voltage command. For example, the command generator 21 updates avoltage value of a voltage command every specified updating cycle basedon one or more specified conditions.

The PWM signal generator 22 generates a PWM signal pursuant to thevoltage command and outputs the PWM signal to the power converter 10.The PWM signal generator 22 may have a first mode and a second mode as aPWM control mode and select one of the first mode and the second modebased on specified conditions.

For example, the PWM signal generator 22 selects the first mode if thetemperature of the power converter 10 is lower than a predeterminedvalue and selects the second mode if the temperature of the powerconverter 10 is equal to or higher than the predetermined value. In thesecond mode, the number of turn-on times of a PWM pulse, namely thenumber of switching times, becomes equal to one half of that availablein the first mode. This makes it possible to reduce a switching lossgenerated in the power converter 10.

FIGS. 1B and 1C are explanatory views of the first mode and the secondmode, respectively. As shown in FIG. 1B, in case of first mode, for each(and preferably, every) voltage command updating cycle Ts, the PWMsignal generator 22 repeatedly outputs to the power converter 10 a PWMsignal having a pattern which is set to sequentially cause a firstperiod T1, a second period T2, and a first period T1 in that order. Thefirst period T1 is a period during which a zero voltage is outputted tovia output lines 5 a and 5 b of the power converter 10. The secondperiod T2 is a period during which a non-zero voltage is outputted tovia the output lines 5 a and 5 b.

In the first mode, the timings of a peak (ridge) and a bottom (valley)of a carrier signal are included in the first period T1. By using thesetimings as updating timings TR, the PWM signal generator 22 updates thevoltage command to be compared with the carrier signal.

As shown in FIG. 1C, in case of the second mode, the PWM signalgenerator 22 alternately outputs, for each (and preferably, every)voltage command updating cycle Ts, a PWM signal having a first patternand a PWM signal having a second pattern to the power converter 10,wherein the first pattern is set such that one first period T1 and onesecond period T2 exist within an updating cycle Ts with the first periodT1 followed by the second period T2, and the second pattern is set suchthat one first period T1 and one second period T2 exist within anupdating cycle Ts with the second period T2 followed by the first periodT1.

In the second mode, the timings of a peak and a bottom of a carriersignal are alternately included in the first period T1 or the secondperiod T2. By using these timings as updating timings TR, the PWM signalgenerator 22 updates the voltage command to be compared with the carriersignal. The peak of the carrier signal indicates a position where thevalue of a waveform of the carrier signal becomes largest. The bottom ofthe carrier signal indicates a position where the value of a waveform ofthe carrier signal becomes smallest.

As shown in FIGS. 1B and 1C, in the second mode, the PWM signalgenerator 22 can make the number of turn-on times of a PWM pulse, namelythe number of switching times, equal to one half of that available inthe first mode. This makes it possible to reduce a switching lossgenerated in the power converter 10.

In the PWM signal generator 22, when the PWM control mode is the firstmode and the second mode, the voltage command updating timings TR remainthe same and the voltage command updating cycle Ts remains unchanged.For that reason, it is possible to suppress an increase in the dead timerequired until the voltage command as a voltage is outputted to the load3.

In the descriptions made herein, the first mode and the second mode areprovided as the PWM control mode and an element of the power convertingapparatus, e.g., the controller, is operable in any one of the firstmode and the second mode. However, by executing the second mode, it ispossible to reduce a switching loss while suppressing an increase in thedead time required until the voltage command is outputted. Thus, thepower converting apparatus 1 may be configured to execute only thesecond mode. This holds true in the power converting apparatusesrelating to other embodiments which will be described later.

Now, configuration examples of the power converter 10 and the controller20 of the power converting apparatus 1 relating to the first embodimentwill be described in more detail. Hereinafter, description will be madeon an example in which the power converter 10 converts DC power tosingle-phase AC voltage and outputs the AC voltage to the load 3 and inwhich the controller 20 generates a PWM signal by a carrier comparisonmethod.

[1.2. Power Converter 10]

FIG. 2 is a view showing configuration examples of the power converter10 and the controller 20. As shown in FIG. 2, the power converter 10 mayinclude input terminals Tp and Tn, output terminals Ta and Tb, asingle-phase inverter circuit 13, a gate drive circuit 11, a currentdetector 12 and a temperature detector 18.

The input terminal Tp is connected to a positive electrode of the powersource 2 while the input terminal Tn is connected to a negativeelectrode of the power source 2. The output terminals Ta and Tb areconnected to the load 3. The power source 2 is a DC power source. Theload 3 is, e.g., a single-phase AC motor.

The single-phase inverter circuit 13 may include switching elements Q1to Q4 and a capacitor C1. The switching elements Q1 to Q4 arebridge-connected to one another and are connected to the load 3 throughthe output terminals Ta and Tb. A protective rectifying element isparallel-connected to each of the switching elements Q1 to Q4. Theswitching elements Q1 to Q4 may be, e.g., a semiconductor device such asan IGBT (Insulated Gate Bipolar Transistor), a MOSFET(Metal-Oxide-Semiconductor Field-Effect Transistor) or the like.

The gate drive circuit 11 amplifies PWM signals L1, L2, R1, and R2outputted from the controller 20 and outputs the amplified PWM signalsL1, L2, R1, and R2 to the gates of the switching elements Q1 to Q4.Thus, the power converter 10 converts the DC voltage inputted from thepower source 2 through the input terminals Tp and Tn to an AC voltageusing the switching operations of the switching elements Q1 to Q4 andoutputs the converted AC voltage to the load 3 through the outputterminals Ta and Tb.

The current detector 12 detects a current (hereinafter referred to as“current I”) flowing between the power converter 10 and the load 3. Thecurrent detector 12 may be, e.g., a current sensor which makes use of aHall element as a magneto-electric conversion element. The temperaturedetector 18 detects, e.g., a temperature (hereinafter referred to as“detection temperature Tc”) within or around the power converter 10.

[1.3. Controller 20]

As shown in FIG. 2, the controller 20 may include a command generator 21and a PWM signal generator 22. The command generator 21 may include acurrent command generator 23 and a current controller 24. The PWM signalgenerator 22 may include a carrier signal generator 30, a mode switcher31, a command updater 32, a shifter 33, and a comparator 34.

The current command generator 23 generates a current command I*. Thecurrent controller 24 generates a voltage command V* such that adeviation between the current command I* and the output current Ibecomes zero.

The carrier signal generator 30 generates and outputs carrier signalsVc1 and Vc2. The carrier signals Vc1 and Vc2 are signals whose positiveand negative polarities are inverted with each other. The carriersignals Vc1 and Vc2 are triangular wave signals but may be, e.g.,saw-tooth wave signals.

The mode switcher 31 outputs a mode signal Sm to the shifter 33 andswitches the first mode and the second mode based on one or moreconditions which may be preset. For example, if the detectiontemperature Tc is lower than a predetermined value, the mode switcher 31outputs a mode signal Sm indicative of the first mode to the shifter 33.If the detection temperature Tc is equal to or higher than thepredetermined value, the mode switcher 31 outputs a mode signal Smindicative of the second mode to the shifter 33.

The command updater 32 inputs the carrier signals Vc1 and Vc2 and thevoltage command V*. By using the timings of the peak and the bottom ofthe carrier signals Vc1 and Vc2 as the updating timings TR, the commandupdater 32 updates, every updating timing TR, the voltage command V*outputted to the comparator 34. Thus, the command updater 32 can outputthe voltage command V* generated by the command generator 21 after theupdating timing TR to the comparator 34 at the next updating timing TR.

The shifter disclosed herein is allowed to relatively shift, withrespect to one or more carrier signals generated from the carrier signalgenerator, one or more phase voltage commands which are to be comparedby the comparator, wherein the phase voltage commends are relativelyshifted based on one of a peak value and a bottom value of the carriersignals.

For instance, if the mode signal Sm indicative of the first mode isoutputted from the mode switcher 31, the shifter 33 relating to thepresent embodiment directly outputs the carrier signals Vc1 and Vc2acquired from the command updater 32, as carrier signals Vc1′ and Vc2′,without shifting the carrier signals Vc1 and Vc2.

On the other hand, if the mode signal Sm indicative of the second modeis outputted from the mode switcher 31, the shifter 33 shifts thecarrier signals Vc1 and Vc2 based on the voltage command V* updated bythe command updater 32 and outputs the shifted carrier signals Vc1 andVc2 as carrier signals Vc1′ and Vc2′. For example, the shifter 33 shiftsone of the carrier signals Vc1 and Vc2 so as to coincide with a peakvalue or a bottom value and shifts the other in the reverse direction.

The comparator 34 compares the carrier signals Vc1′ and Vc2′ with thevoltage command V* and generates PWM signals L1, L2, R1, and R2 based ona result of the comparison. The comparator 34 outputs the PWM signalsL1, L2, R1, and R2 to the gate drive circuit 11.

Now, the relationship between the carrier signals Vc1 and Vc2, thecarrier signals Vc1′ and Vc2′ and the voltage command V* will bedescribed in detail with reference to FIGS. 3A to 3C.

FIG. 3A is an explanatory view of the first mode. As shown in FIG. 3A,in case of the first mode, the comparator 34 of the PWM signal generator22 compares the carrier signals Vc1′ and Vc2′ having the same values asthe carrier signals Vc1 and Vc2 with the voltage command V* andgenerates PWM signals L1, L2, R1, and R2. Thus, every updating cycle Tsof the voltage command V*, the PWM signal generator 22 repeatedlyoutputs a PWM signal having a control pattern in which the PWM signalmigrates in the order of a first period T1, a second period T2 and afirst period T1.

FIG. 3B is an explanatory view of the second mode in case where thevoltage command V* is positive. If the voltage command V* is positiveand if the PWM control mode is the second mode, the shifter 33 of thePWM signal generator 22 finds a difference ΔVcp between the peak valueVp of the carrier signals Vc1 and Vc2 and the updated voltage commandV*.

The shifter 33 generates a carrier signal Vc1′ by subtracting thedifference ΔVcp from the carrier signal Vc1 and generates a carriersignal Vc2′ by adding the difference ΔVcp to the carrier signal Vc2. Thecomparator 34 compares the voltage command V* updated by the commandupdater 32 with the carrier signals Vc1′ and Vc2′ outputted from theshifter 33 and outputs the comparison results as PWM signals L1, L2, R1,and R2.

In the example shown in FIG. 3B, the PWM signal generator 22 makes surethat the bottom of the carrier signal Vc2 exists within the secondperiod T2. Alternatively, the PWM signal generator 22 may make sure thatthe bottom of the carrier signal Vc2 exists within the first period T1.In this case, the PWM signal generator 22 generates PWM signals L1 andL2 by comparing the carrier signal Vc1' with the voltage command V* andgenerates PWM signals R1 and R2 by comparing the carrier signal Vc2′with the voltage command V*.

FIG. 3C is an explanatory view of the second mode in case where thevoltage command V* is negative. If the voltage command V* is negativeand if the PWM control mode is the second mode, the shifter 33 of thePWM signal generator 22 finds a difference ΔVcb between the bottom valueVb of the carrier signals Vc1 and Vc2 and the updated voltage commandV*.

The shifter 33 generates a carrier signal Vc1′ by adding the differenceΔVcb to the carrier signal Vc1 and generates a carrier signal Vc2′ bysubtracting the difference ΔVcb from the carrier signal Vc2. Thecomparator compares the voltage command V* updated by the commandupdater 32 with the carrier signals Vc1′ and Vc2′ outputted from theshifter 33 and outputs the comparison results as PWM signals L1, L2, R1,and R2.

In the example shown in FIG. 3C, the PWM signal generator 22 makes surethat the bottom of the carrier signal Vc2 exists within the first periodT1. Alternatively, the PWM signal generator 22 may make sure that thebottom of the carrier signal Vc2 exists within the second period T2. Inthis case, the PWM signal generator 22 generates PWM signals L1 and L2by comparing the carrier signal Vc1′ with the voltage command V* andgenerates PWM signals R1 and R2 by comparing the carrier signal Vc2′with the voltage command V*.

As described above, in case of the second mode, every updating cycle Tsof the voltage command V*, the PWM signal generator 22 alternatelyoutputs a PWM signal having a control pattern in which the PWM signalmigrates in the order of a first period T1 and a second period T2 duringone updating cycle Ts and a PWM signal having a control pattern in whichthe PWM signal migrates in the order of a second period T2 and a firstperiod T1 during one updating cycle Ts. Thus, as shown in FIG. 1B, inthe second mode, the PWM signal generator 22 can make the number ofturn-on times of a PWM pulse, namely the number of switching times,equal to one half of that available in the first mode. This makes itpossible to reduce a switching loss generated in the power converter 10.

Furthermore, the PWM signal generator 22 does not change the updatingcycle Ts of the voltage command V* in any of the first and second modes.It is therefore possible to suppress an increase in the dead timerequired until the voltage command V* as a voltage is outputted to theload 3. Therefore, as compared with a case where the dead time isallowed to become longer, it is possible to increase the gain of thecurrent controller 24 and to perform current control with highresponsiveness.

FIG. 4 is a view showing a configuration example of the comparator 34.As shown in FIG. 4, the comparator 34 includes comparators 41 and 42 andNOT circuits 43 and 44. The comparator 41 compares the voltage commandV* with the carrier signal Vc1′. If the voltage command V* is equal toor higher than the carrier signal Vc1′, the comparator 41 outputs a highlevel signal. If the voltage command V* is lower than the carrier signalVc1′, the comparator 41 outputs a low level signal.

The comparator 42 compares the voltage command V* with the carriersignal Vc2′. If the voltage command V* is equal to or higher than thecarrier signal Vc2′, the comparator 42 outputs a high level signal. Ifthe voltage command V* is lower than the carrier signal Vc2′, thecomparator 42 outputs a low level signal. The NOT circuit 43 inverts theoutput of the comparator 41 and outputs the inverted output of thecomparator 41. The NOT circuit 44 inverts the output of the comparator42 and outputs the inverted output of the comparator 42.

The comparator 34 outputs the output of the comparator 41 as a PWMsignal R1 and outputs the output of the NOT circuit 43 as a PWM signalR2. Furthermore, the comparator 34 outputs the output of the comparator42 as a PWM signal L1 and outputs the output of the NOT circuit 44 as aPWM signal L2.

The configuration of the comparator 34 is not limited to theconfiguration shown in FIG. 4. As an alternative example, it may bepossible to employ a configuration in which four comparators areinstalled so as to output PWM signals L1, L2, R1, and R2.

[1.4. Processing in Controller 20]

Now, description will be made on one example of the processing flow ofthe controller 20. FIG. 5 is a flowchart illustrating one example of theprocessing flow of the controller 20.

As shown in FIG. 5, the command generator 21 of the controller 20generates a voltage command V* (step S11). Then, the PWM signalgenerator 22 of the controller 20 determines whether now is the updatingtiming TR of the voltage command V* (step S12). If it is determined thatnow is not the updating timing TR of the voltage command V* (if No atstep S12), the PWM signal generator 22 repeatedly performs theprocessing of step S12.

If it is determined that now is the updating timing TR of the voltagecommand V* (if Yes at step S12), the PWM signal generator 22 determineswhether it is the second mode (step S13). For example, if the voltagecommand V* is smaller than a predetermined value, the PWM signalgenerator 22 determines that it is the second mode.

If it is determined that it is the second mode (if Yes at step S13), thePWM signal generator 22 relatively shifts the voltage command V* withrespect to the carrier signals Vc1 and Vc2 (step S14). For example, thePWM signal generator 22 shifts the carrier signals Vc1 and Vc2 dependingon the difference ΔVcp (or the difference ΔVcb) between the peak valueVp (or the bottom value Vb) of the carrier signals Vc1 and Vc2 and thevoltage command V*. Thus, the voltage command V* is relatively shiftedwith respect to the carrier signals Vc1 and Vc2.

If the processing of step S14 is completed or if it is determined atstep S13 that it is not the second mode (if No at step S13), the PWMsignal generator 22 compares the voltage command V* with the carriersignals Vc1′ and Vc2′ and generates PWM signals L1, L2, R1, and R2 (stepS15).

Meanwhile, the method for controlling the power converting apparatusrelating to the present embodiment may include, e.g., a commandgenerating process and a signal generating process, and correspond tothe processing follow of the controller 20 and/or its elements asdescribed above. Specifically, the processing flow at step S11 may be anexample or an element of the command generating process, and theprocessing follow at steps S12 to S15 may be an example or an element ofthe signal generating process.

The controller 20 may also be an example of the control device relatingto the present embodiment and include a microcomputer including, e.g., aCPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random

Access Memory), an input/output port, and the like, and various kinds ofcircuits. The CPU of the microcomputer realizes control of the commandgenerator 21 and the PWM signal generator 22 by reading out andexecuting a program stored in the ROM. At least one or all of thecommand generator 21 and the PWM signal generator 22 may be configuredby the hardware such as an ASIC (Application Specific IntegratedCircuit), an FPGA (Field Programmable Gate Array) or the like.

As described above, every updating cycle Ts of the voltage command V*,the power converting apparatus 1 relating to the first embodimentoutputs to the power converter 10 a PWM signal in which one first periodT1 and one or more second periods T2 are combined with each other. Thus,the power converting apparatus 1 can suppress an increase in the deadtime required until the voltage command V* outputted from the commandgenerator 21 is outputted as a voltage (a voltage pursuant to thevoltage command V*) from the power converter 10.

[2. Second Embodiment]

Next, description will be made on a power converting apparatus relatingto a second embodiment. The power converting apparatus relating to thesecond embodiment differs from the power converting apparatus 1 relatingto the first embodiment in that the power converting apparatus of thesecond embodiment is a serial multiplex power converter which converts aDC voltage to a three-phase AC voltage and outputs the three-phase ACvoltage. In the following description, the constituent elements havingthe same functions as those of the power converting apparatus 1 will bedesignated by like reference symbols. No duplicate description will bemade thereon.

FIG. 6 is a view showing a configuration example of a power convertingapparatus 1A relating to the second embodiment. The power convertingapparatus 1A includes a power converting cell unit 9, a current detector12A and a command generator 21A and outputs a three-phase AC current toa load 3A (e.g., a three-phase AC motor or a power system).

As shown in FIG. 6, the power converting cell unit 9 includes ninesingle-phase power converting cells 15 a to 15 i (hereinafter oftengenerically referred to as “single-phase power converting cell 15”). Thesingle-phase power converting cells 15 a to 15 i are divided into threegroups of single-phase power converting cells in a correspondingrelationship with the U-phase, V-phase, and W-phase of the load 3A.

Specifically, one end of a single-phase power converting cell groupconfigured by serially connecting the output terminals of thesingle-phase power converting cells 15 a to 15 c is connected to aneutral point N while the other end thereof is connected to a U-phaseterminal of the load 3A. Furthermore, one end of a single-phase powerconverting cell group configured by serially connecting the outputterminals of the single-phase power converting cells 15 d to 15 f isconnected to the neutral point N while the other end thereof isconnected to a V-phase terminal of the load 3A. Moreover, one end of asingle-phase power converting cell group configured by seriallyconnecting the output terminals of the single-phase power convertingcells 15 g to 15 i is connected to the neutral point N while the otherend thereof is connected to a W-phase terminal of the load 3A.

The current detector 12A detects phase currents Iu, Iv, and Iw(hereinafter referred to as “output phase current Iuvw”) which flowbetween the power converting cell unit 9 and the U-phase, V-phase, andW-phase terminals of the load 3A. The current detector 12A may be, e.g.,a current sensor which makes use of a Hall element as a magneto-electricconversion element.

FIG. 7 is a view showing a configuration example of the single-phasepower converting cell 15. As shown in FIG. 7, the single-phase powerconverting cell 15 includes a power converter 10A, a controller 17 and atemperature detector 18. The power converter 10A includes a gate drivecircuit 11 and a single-phase inverter circuit 13. Only one temperaturedetector 18 may be provided with respect to the nine single-phase powerconverting cells 15 a to 15 i.

The single-phase power converting cell 15 includes input terminals Td(input terminals Tp and Tn) and output terminals Ta and Tb. Thesingle-phase power converting cell 15 converts a DC voltage inputtedfrom the power source 2 to the input terminals Td, to a single-phase ACvoltage and outputs the single-phase AC voltage through the outputterminals Ta and Tb.

For example, the power converter 10A outputs a voltage obtained byadding up the output voltages of the single-phase power converting cells15 a to 15 c, to the U-phase terminal of the load 3A. The powerconverter 10A outputs a voltage obtained by adding up the outputvoltages of the single-phase power converting cells 15 d to 15 f, to theV-phase terminal of the load 3A. The power converter 10A outputs avoltage obtained by adding up the output voltages of the single-phasepower converting cells 15 g to 15 i, to the W-phase terminal of the load3A.

The controller 17 includes a PWM signal generator 22. The PWM signalgenerator 22 generates PWM signals L1, L2, R1, and R2 based on thebelow-mentioned phase voltage command outputted from the commandgenerator 21A.

The command generator 21A includes a current command generator 23A and acurrent controller 24A. The current command generator 23A generates aphase current command Iuvw*. The phase current command Iuvw* includesphase current commands Iu*, Iv*, and Iw*. The current controller 24Agenerates a phase voltage command Vuvw* such that a deviation betweenthe phase current command Iuvw* and the output phase current Iuvwbecomes zero. The phase voltage command Vuvw* includes phase voltagecommands Vu*, Vv*, and Vw* which are U-phase, V-phase, and W-phasevoltage commands. The command generator 21A outputs the phase voltagecommand Vu* as a voltage command V* to the single-phase power convertingcells 15 a to 15 c. The command generator 21A outputs the phase voltagecommand Vv* as a voltage command V* to the single-phase power convertingcells 15 d to 15 f. The command generator 21A outputs the phase voltagecommand Vw* as a voltage command V* to the single-phase power convertingcells 15 g to 15 f.

As described above, each of the single-phase power converting cells 15 ato 15 i of the power converting apparatus 1A relating to the secondembodiment includes the PWM signal generator 22. Accordingly, just likethe power converting apparatus 1 relating to the first embodiment, inthe second mode, the PWM signal generator 22 can make the number ofturn-on times of a PWM pulse, namely the number of switching times,equal to one half of that available in the first mode. This makes itpossible to reduce a switching loss while suppress an increase in thedead time.

The flow of the processing in the command generator 21A is the same asthe flow of the processing of step S11 shown in FIG. 5. The flow of theprocessing in the controller 17 is the same as the flow of theprocessing of steps S12 to S15 shown in FIG. 5. Therefore, no furtherdetailed description will be made thereon.

Meanwhile, the method for controlling the power converting apparatusrelating to the present embodiment may include, e.g., a commandgenerating process and a signal generating process, and correspond tothe processing follow of the controller 17 and/or its elements asdescribed above. Specifically, the flow of the processing in thecontroller 17 may be an example or an element of the command generatingprocess, and the flow of the processing in the command generator 21A maybe an example or an element of the signal generating process.

Further, each of the command generator 21A and the PWM signal generator22 may also be examples of elements of the control device relating tothe present embodiment, and, similarly to the controller 20, include amicrocomputer and various kinds of circuits. The CPU of themicrocomputer realizes control of the command generator 21A and the PWMsignal generator 22 by reading out and executing a program stored in theROM. One or all of the command generator 21A and the PWM signalgenerator 22 may be configured by the hardware such as an ASIC, an FPGAor the like.

[3. Third Embodiment]

Next, description will be made on a power converting apparatus relatingto a third embodiment. The power converting apparatus relating to thethird embodiment differs from the power converting apparatus 1 relatingto the first embodiment in that the power converting apparatus of thethird embodiment converts a DC voltage to a three-phase AC voltage andoutputs the three-phase AC voltage. In the following description, theconstituent elements having the same functions as those of the powerconverting apparatuses 1 and 1A will be designated by like referencesymbols. No duplicate description will be made thereon.

FIG. 8 is a view showing a configuration example of a power convertingapparatus 1B relating to the third embodiment. The power convertingapparatus 1B includes a power converter 10B and a controller 20B andoutputs a three-phase AC current to a load 3A. Configuration examples ofthe power converter 10B and the controller 20B will now be described indetail.

[3.1. Power Converter 10B]

As shown in FIG. 8, the power converter 10B includes input terminals Tpand Tn, output terminals Tu, Tv, and Tw, a three-phase two-levelinverter circuit 13B, a gate drive circuit 11B, a current detector 12Aand a temperature detector 18. The output terminals Tu, Tv, and Tw arerespectively connected to the U-phase, V-phase, and W-phase terminals ofthe load 3A.

The three-phase two-level inverter circuit 13B includes switchingelements Q11 to Q16 and a capacitor C1. The switching elements Q11 toQ16 are bridge-connected to one another and are connected to the load 3Athrough the output terminals Tu, Tv, and Tw. A protective rectifyingelement is parallel-connected to each of the switching elements Q11 toQ16. The switching elements Q11 to Q16 may be, e.g., a semiconductordevice such as an IGBT, a MOSFET or the like.

The gate drive circuit 11B generates gate signals S1 to S6 based on thePWM signals PA, PB, and PC outputted from the controller 20B. Forexample, the gate drive circuit 11B outputs the gate signals S1, S3, andS5 obtained by amplifying the PWM signals PA, PB, and PC, to theswitching elements Q11, Q13, and Q15. Furthermore, the gate drivecircuit 11B outputs the gate signals S2, S4, and S6 obtained byinverting and amplifying the PWM signals PA, PB, and PC, to theswitching elements Q12, Q14, and Q16.

Thus, the power converter 10B converts the DC voltage inputted from thepower source 2 through the input terminals Tp and Tn to the three-phaseAC voltage using the switching operations of the switching elements Q11to Q16 and outputs the converted three-phase AC voltage fourth member M4the output terminals Tu, Tv, and Tw to the load 3A through the outputlines 6 a to 6 c.

[3.2. Controller 20B]

As shown in FIG. 8, the controller 20B includes a command generator 21Aand a PWM signal generator 22B. The PWM signal generator 22B includes acarrier signal generator 30B, a mode switcher 31B, a command updater32B, a shifter 33B, and a comparator 34B.

The carrier signal generator 30B outputs a carrier signal Vc. Thecarrier signal Vc is a triangular wave signal but may be, e.g., asaw-tooth wave signal.

The mode switcher 31B outputs a mode signal Sm to the shifter 33B andswitches the first mode and the second mode. For example, if thedetection temperature Tc is lower than a predetermined value, the modeswitcher 31B outputs a mode signal Sm indicative of the first mode tothe shifter 33B. If the detection temperature Tc is equal to or higherthan the predetermined value, the mode switcher 31B outputs a modesignal Sm indicative of the second mode to the shifter 33B.

The command updater 32B inputs the carrier signal Vc and the phasevoltage command Vuvw*. By using the timings of the peak and the bottomof the carrier signal Vc as the updating timings TR, the command updater32B updates, every updating timing TR, the phase voltage command Vuvw*outputted to the comparator 34B. Thus, the command updater 32B canoutput the phase voltage command Vuvw* generated by the commandgenerator 21A after the updating timing TR to the comparator 34B at thenext updating timing TR.

If the mode signal Sm indicative of the first mode is outputted from themode switcher 31B, the shifter 33B directly outputs the phase voltagecommand Vuvw* acquired from the command updater 32B, as a phase voltagecommand Vuvw1*, without shifting the phase voltage command Vuvw*. Thephase voltage command Vuvw1* includes phase voltage commands Vu1*, Vv1*,and Vw1* which are U-phase, V-phase, and W-phase voltage commands.

On the other hand, if the mode signal Sm indicative of the second modeis outputted from the mode switcher 31B, the shifter 33B shifts thephase voltage command Vuvw*, based on the phase voltage command Vuvw*updated by the command updater 32B and the carrier signal Vc, andoutputs the shifted phase voltage command Vuvw* as a phase voltagecommand Vuvw1*.

The comparator 34B compares the carrier signal Vc with the phase voltagecommands Vu1*, Vv1*, and Vw1* and generates PWM signals PA, PB, and PCbased on a result of the comparison.

For example, if the voltage value of the carrier signal Vc is equal toor larger than the voltage value of the phase voltage command Vu1*, thecomparator 34B outputs a PWM signal PA having a low level. If thevoltage value of the carrier signal Vc is smaller than the voltage valueof the phase voltage command Vu1*, the comparator 34B outputs a PWMsignal PA having a high level.

Similarly, if the voltage value of the carrier signal Vc is equal to orlarger than the voltage value of the phase voltage command Vv1*, thecomparator 34B outputs a PWM signal PB having a low level. If thevoltage value of the carrier signal Vc is smaller than the voltage valueof the phase voltage command Vv1*, the comparator 34B outputs a PWMsignal PB having a high level.

Moreover, if the voltage value of the carrier signal Vc is equal to orlarger than the voltage value of the phase voltage command Vw1*, thecomparator 34B outputs a PWM signal PC having a low level. If thevoltage value of the carrier signal Vc is smaller than the voltage valueof the phase voltage command Vw1*, the comparator 34B outputs a PWMsignal PC having a high level.

The comparator 34B outputs the generated PWM signals PA, PB, and PC tothe gate drive circuit 11B.

The relationship between the carrier signal Vc, the phase voltagecommand Vuvw* and the phase voltage command Vuvw1* will now be describedin detail with reference to FIGS. 9A to 9C. FIG. 9A is an explanatoryview of the first mode.

As shown in FIG. 9A, in case of the first mode, the comparator 34B ofthe PWM signal generator 22B compares the phase voltage commands Vu1*,Vv1*, and Vw1* having the same values as the phase voltage commands Vu*,Vv*, and Vw* with the carrier signal Vc and generates the PWM signalsPA, PB, and PC. Thus, every updating cycle Ts of the phase voltagecommand Vuvw*, the PWM signal generator 22B repeatedly outputs a PWMsignal having a control pattern in which the PWM signal migrates in theorder of a first period T1, a second period T2, and a first period T1.

FIG. 9B is an explanatory view showing one example of the second mode.The shifter relating to the present embodiment is allowed to shift thephase voltage commands such that the greatest one among the phasevoltage commands becomes the peak value of the carrier signal or suchthat the smallest one among the phase voltage commands becomes thebottom value of the carrier signal.

For instance, if the mode signal Sm indicates the second mode, theshifter 33B of the PWM signal generator 22B finds, as shown in FIG. 9B,a difference ΔVc1 between the largest phase voltage command among thephase voltage commands Vu*, Vv*, and Vw* and the peak value Vp (see FIG.9A) of the carrier signal Vc.

The shifter 33B generates phase voltage commands Vu1*, Vv1*, and Vw1* byadding the difference ΔVc1 to the phase voltage commands Vu*, Vv*, andVw*. The comparator 34B compares the phase voltage commands Vu1*, Vv1*,and Vw1* with the carrier signal Vc and outputs the comparison resultsas PWM signals PA, PB, and PC.

FIG. 9C is an explanatory view showing another example of the secondmode. As shown in FIG. 9C, if the mode signal Sm indicates the secondmode, the shifter 33B of the PWM signal generator 22B finds a differenceΔVc2 between the smallest phase voltage command among the phase voltagecommands Vu*, Vv*, and Vw* and the bottom value Vb (see FIG. 9A) of thecarrier signal Vc.

The shifter 33B generates phase voltage commands Vu1*, Vv1*, and Vw1* bysubtracting the difference ΔVc2 from the phase voltage commands Vu*,Vv*, and Vw*. The comparator 34B compares the phase voltage commandsVu1*, Vv1*, and Vw1* with the carrier signal Vc and outputs thecomparison results as PWM signals PA, PB, and PC.

As described above, the PWM signal generator 22B relating to the thirdembodiment relatively shifts the phase voltage commands Vu*, Vv*, andVw* with respect to the carrier signal Vc, based on the peak value Vp orthe bottom value Vb of the carrier signal Vc, and compares the shiftedphase voltage commands Vu*, Vv*, and Vw* with the carrier signal Vc.

Thus, in the second mode, every updating cycle Ts of the phase voltagecommand Vuvw*, the PWM signal generator 22B can alternately output a PWMsignal having a control pattern in which the PWM signal migrates in theorder of a first period T1 and a second period T2 during one updatingcycle Ts and a PWM signal having a control pattern in which the PWMsignal migrates in the order of a second period T2 and a first period T1during one updating cycle Ts.

For that reason, in the second mode, the PWM signal generator 22B canmake the number of turn-on times of a PWM pulse, namely the number ofswitching times, equal to two thirds of that available in the firstmode. This makes it possible to reduce a switching loss generated in thepower converter 10B. Furthermore, the PWM signal generator 22B does notchange the updating cycle Ts of the phase voltage command Vuvw* in anyof the first and second modes. It is therefore possible to suppress anincrease in the dead time required until the phase voltage commands Vu*,Vv*, and Vw* as U-phase, V-phase, and W-phase output voltages areoutputted to the load 3A. Therefore, as compared with a case where thedead time is allowed to become longer, it is possible to increase thegain of the current controller 24A and to perform current control withhigh responsiveness.

Furthermore, if the mode signal Sm indicates the second mode, theshifter 33B of the PWM signal generator 22B may alternately switch theprocessing shown in FIG. 9B and the processing shown in FIG. 9C, forexample, each time when the power converting apparatus 1B is started up.

The flow of the processing in the controller 20B is the same as the flowof the processing shown in FIG. 5. Therefore, no further detaileddescription will be made thereon.

Meanwhile, the method for controlling the power converting apparatusrelating to the present embodiment may include, e.g., a commandgenerating process and a signal generating process, and correspond tothe processing follow of the controller 20B and/or its elements asdescribed above. Specifically, the processing flow at step S11 as shownin FIG. 5 may be an example or an element of the command generatingprocess, and the processing follow at steps S12 to S15 as shown in FIG.5 may be an example or an element of the signal generating process.

Further, the controller 20B may also be an example of the control devicerelating to the present embodiment, and, similarly to the controller 20,include a microcomputer and various kinds of circuits. The CPU of themicrocomputer realizes control of the command generator 21A and the PWMsignal generator 22B by reading out and executing a program stored inthe ROM. One or all of the command generator 21A and the PWM signalgenerator 22B may be configured by the hardware such as an ASIC, an FPGAor the like.

[4. Fourth Embodiment]

Next, description will be made on a power converting apparatus relatingto a fourth embodiment. The power converting apparatus relating to thefourth embodiment differs from the power converting apparatus 1Brelating to the third embodiment in that the power converting apparatusof the fourth embodiment converts a three-level DC voltage to athree-phase AC voltage. In the following description, the constituentelements having the same functions as those of the power convertingapparatus 1B will be designated by like reference symbols. No duplicatedescription will be made thereon.

FIG. 10 is a view showing a configuration example of a power convertingapparatus 1C relating to the fourth embodiment. The power convertingapparatus 1C includes a power converter 10C and a controller 20C andoutputs a three-phase AC current to a load 3A. Configuration examples ofthe power converter 10C and the controller 20C will now be described indetail.

[4.1. Power Converter 10C]

The power converter relating to the present embodiment may be configuredas a multilevel output. That is, the power converter may be configuredto output the voltage at multiple levels. As shown in FIG. 10, the powerconverter 10C includes input terminals Tp and Tn, output terminals Tu,Tv, and Tw, a multilevel inverter circuit(e.g., a three-phasethree-level inverter circuit 13C), a gate drive circuit 11C, a currentdetector 12A and a temperature detector 18.

The three-phase three-level inverter circuit 13C includes switchingelements Q21 to Q24, Q31 to Q34, and Q41 to Q44, capacitors C21 and C22,and diodes D21 to D26. The three-phase three-level inverter circuit 13Cis connected to the load 3A through the output terminals Tu, Tv, and Tw.A protective rectifying element is parallel-connected to each of theswitching elements Q21 to Q24, Q31 to Q34, and Q41 to Q44. The switchingelements Q21 to Q24, Q31 to Q34, and Q41 to Q44 may be, e.g., asemiconductor device such as an IGBT, a MOSFET or the like.

The gate drive circuit 11C generates gate signals PA1 to PA4, PB1 toPB4, and PC1 to PC4 based on the PWM signals PA, PB, and PC outputtedfrom the controller 20C. Description will now be made on an example inwhich the gate drive circuit 11C generates the gate signals PA1 to PA4using the PWM signal PA.

For example, if the PWM signal PA is of a high level, the gate drivecircuit 11C keeps the gate signals PA1 and PA2 at a high level and keepsthe gate signals PA3 and PA4 at a high level. Thus, the three-phasethree-level inverter circuit 13C outputs a DC voltage (hereinafterreferred to as “power source voltage Vdc”) of the power source 2 fromthe U-phase output terminal Tu.

For example, if the PWM signal PA is of a low level, the gate drivecircuit 11C keeps the gate signals PA3 and PA4 at a high level and keepsthe gate signals PA1 and PA2 at a low level. Thus, the three-phasethree-level inverter circuit 13C outputs a zero potential (groundpotential) from the U-phase output terminal Tu.

For example, if the PWM signal PA is of a middle level, the gate drivecircuit 11C keeps the gate signals PA2 and PA3 at a high level and keepsthe gate signals PA1 and PA4 at a low level. Thus, the three-phasethree-level inverter circuit 13C outputs a voltage (Vdc/2) equal to onehalf of the power source voltage Vdc from the U-phase output terminalTu.

With respect to the PWM signals PB and PC, just like the PWM signal PA,the gate drive circuit 11C generates gate signals PB1 to PB4 and PC1 toPC4. In this way, the power converter 10C converts the DC voltageinputted from the power source 2 through the input terminals Tp and Tnto the three-phase AC voltage using the switching operations of theswitching elements Q21 to Q24, Q31 to Q34, and Q41 to Q44 and outputsthe three-phase AC voltage from the output terminals Tu, Tv, and Tw tothe load 3A through the output lines 6 a to 6 c.

[4.2. Controller 20C]

As shown in FIG. 10, the controller 20C includes a command generator 21Cand a PWM signal generator 22C. The command generator 21C includes acurrent command generator 23A, a current controller 24C, and a voltagecommand generator 25C.

The current controller 24C generates a voltage command vector Vs* suchthat a deviation between the phase current command Iuvw* and the outputphase current Iuvw becomes zero. The voltage command generator 25Cgenerates and outputs a phase voltage command Vuvwpn from the voltagecommand vector Vs*. The phase voltage command relating to the presentembodiment may include first phase voltage command and second phasevoltage command with respect to each phase. Accordingly, in a case of amultiphase AC voltage, a plurality of first phase voltage commands and aplurality of second phase voltage commands may exist. For instance, thephase voltage command Vuvwpn* includes first phase voltage commandsVup*, Vvp*, and Vwp* and second phase voltage commands Vun*, Vvn*, andVwn* corresponding to the U-phase, the V-phase, and W-phase. The voltagecommand generator 25C generates the phase voltage command Vuvwpn* fromthe voltage command vector Vs* using, e.g., well-known dipolarmodulation (see, e.g., Japanese Patent Application Publication No.05-211775).

The PWM signal generator 22C includes a carrier signal generator 30B, amode switcher 31B, a command updater 32C, a shifter 33C and a comparator34C.

The command updater 32C inputs the carrier signal Vc and the phasevoltage command Vuvwpn*. By using the timings of the peak and the bottomof the carrier signal Vc as the updating timings TR, the command updater32C updates, every updating timing TR, the phase voltage command Vuvwpn*outputted to the comparator 34C. Thus, the command updater 32C canoutput the phase voltage command Vuvwpn* generated by the voltagecommand generator 25C after the updating timing TR to the comparator 34Cat the next updating timing TR.

If the mode signal Sm indicative of the first mode is outputted from themode switcher 31B, the shifter 33C directly outputs the phase voltagecommand Vuvwpn* acquired from the command updater 32C, as a phasevoltage command Vuvwpn1*, without shifting the phase voltage commandVuvwpn*. The phase voltage command Vuvwpn1* includes first phase voltagecommands Vup1*, Vvp1*, and Vwp1* and second phase voltage commandsVun1*, Vvn1*, and Vwn1* corresponding to the U-phase, the V-phase, andthe W-phase.

On the other hand, if the mode signal Sm indicative of the second modeis outputted from the mode switcher 31B, the shifter 33C shifts thephase voltage command Vuvwpn*, based on the phase voltage commandVuvwpn* updated by the command updater 32C and the carrier signal Vc,and outputs the shifted phase voltage command Vuvwpn* as a phase voltagecommand Vuvwpn1*.

The comparator 34C compares the carrier signal Vc with the phase voltagecommands Vup1*, Vvp1*, Vwp1*, Vun1*, Vvn1*, and Vwn1* and generates PWMsignals PA, PB, and PC.

For example, if the voltage value of the carrier signal Vc is equal toor larger than the voltage value of the phase voltage command Vup1*, thecomparator 34C outputs a PWM signal PA having a low level. If thevoltage value of the carrier signal Vc is between the voltage value ofthe phase voltage command Vup1* and the voltage value of the phasevoltage command Vun1*, the comparator 34C outputs a PWM signal PA havinga middle level. If the voltage value of the carrier signal Vc is smallerthan the voltage value of the phase voltage command Vun1*, thecomparator 34C outputs a PWM signal PA having a high level.

Similarly, if the voltage value of the carrier signal Vc is equal to orlarger than the voltage value of the phase voltage command Vvp1*, thecomparator 34C outputs a PWM signal PB having a low level. If thevoltage value of the carrier signal Vc is between the voltage value ofthe phase voltage command Vvp1* and the voltage value of the phasevoltage command Vvn1*, the comparator 34C outputs a PWM signal PB havinga middle level. If the voltage value of the carrier signal Vc is smallerthan the voltage value of the phase voltage command Vvn1*, thecomparator 34C outputs a PWM signal PB having a high level.

Moreover, if the voltage value of the carrier signal Vc is equal to orlarger than the voltage value of the phase voltage command Vwp1*, thecomparator 34C outputs a PWM signal PC having a low level. If thevoltage value of the carrier signal Vc is between the voltage value ofthe phase voltage command Vwp1* and the voltage value of the phasevoltage command Vwn1*, the comparator 34C outputs a PWM signal PC havinga middle level. If the voltage value of the carrier signal Vc is smallerthan the voltage value of the phase voltage command Vwn1*, thecomparator 34C outputs a PWM signal PC having a high level.

The comparator 34C outputs the generated PWM signals PA, PB, and PC tothe gate drive circuit 11C.

The relationship between the carrier signal Vc, the phase voltagecommand Vuvwpn* and the phase voltage command Vuvwpn1* will now bedescribed in detail with reference to FIGS. 11A and 11B. FIG. 11A is anexplanatory view of the first mode.

As shown in FIG. 11A, in case of the first mode, the comparator 34C ofthe PWM signal generator 22C compares the phase voltage commands Vup1*,Vvp1*, Vwp1*, Vun1*, Vvn1*, and Vwn1* having the same values as thephase voltage commands Vup*, Vvp*, Vwp*, Vun*, Vvn*, and Vwn* with thecarrier signal Vc and generates the PWM signals PA, PB, and PC.

Thus, for each (and preferably, every) updating cycle Ts of the phasevoltage command Vuvwpn*, the PWM signal generator 22C repeatedly outputsa PWM signal having a control pattern which is set to sequentially causea first period T1, a second period T2, a first period T1, a secondperiod T2, and a first period T1 in that order.

FIG. 11B is an explanatory view showing one example of the second mode.The shifter of the PWM signal generator relating to the presentembodiment shifts the first phase voltage commands such that thegreatest one among the first phase voltage commands becomes the peakvalue of the carrier signal and shift the second phase voltage commandssuch that the smallest one among the second phase voltage commandsbecomes the bottom value of the carrier signal.

Specifically, as shown in FIG. 11B, the shifter 33C of the PWM signalgenerator 22C finds a difference ΔVc3 between the largest phase voltagecommand among the phase voltage commands Vup1*, Vvp1*, and Vwp1* and thepeak value Vp (see FIG. 11A) of the carrier signal Vc. The shifter 33Cgenerates phase voltage commands Vup1*, Vvp1*, and Vwp1* by adding thedifference ΔVc3 to the phase voltage commands Vup*, Vvp*, and Vwp*.

Furthermore, the shifter 33C finds a difference ΔVc4 between thesmallest phase voltage command among the phase voltage commands Vun*,Vvn*, and Vwn* and the bottom value Vb (see FIG. 11A) of the carriersignal Vc. The shifter 33C generates phase voltage commands Vun1*,Vvn1*, and Vwn1* by subtracting the difference ΔVc4 from the phasevoltage commands Vun*, Vvn*, and Vwn*.

The comparator 34C compares the phase voltage commands Vup1*, Vvp1*,Vwp1*, Vun1*, Vvn1*, and Vwn1* with the carrier signal Vc and outputsthe comparison results as PWM signals PA, PB, and PC. Thus, for each(and preferably, every) updating cycle Ts of the phase voltage commandVuvwpn*, the PWM signal generator 22C is allowed to repeatedly output aPWM signal having a control pattern which is set to sequentially cause asecond period T2, a first period T1, and a second period T2 in thatorder.

As described above, the PWM signal generator 22C relating to the fourthembodiment relatively shifts the phase voltage commands Vup*, Vvp*,Vwp*, Vun*, Vvn*, and Vwn* with respect to the carrier signal Vc, basedon the peak value Vp or the bottom value Vb of the carrier signal Vc,and compares the shifted phase voltage commands Vup*, Vvp*, Vwp*, Vun*,Vvn*, and Vwn* with the carrier signal Vc.

Thus, in the second mode, the PWM signal generator 22C can make thenumber of turn-on times of a PWM pulse, namely the number of switchingtimes, equal to two thirds of that available in the first mode. Thismakes it possible to reduce a switching loss generated in the powerconverter 10C. Furthermore, the PWM signal generator 22C does not changethe updating cycle Ts of the phase voltage command Vuvwpn* in any of thefirst and second modes. It is therefore possible to suppress an increasein the dead time required until the phase voltage commands Vuvw* asU-phase, V-phase, and W-phase output voltages are outputted to the load3A. Therefore, as compared with a case where the dead time is allowed tobecome longer, it is possible to increase the gain of the currentcontroller 24A and to perform current control with high responsiveness.

The flow of the processing in the controller 20C is the same as the flowof the processing shown in FIG. 5. Therefore, no further detaileddescription will be made thereon.

Meanwhile, the method for controlling the power converting apparatusrelating to the present embodiment may include, e.g., a commandgenerating process and a signal generating process, and correspond tothe processing follow of the controller 20C and/or its elements asdescribed above. Specifically, the processing flow at step S11 as shownin FIG. 5 may be an example or an element of the command generatingprocess, and the processing follow at steps S12 to S15 as shown in FIG.5 may be an example or an element of the signal generating process.

Further, the controller 20C may also be an example of the control devicerelating to the present embodiment, and, similarly to the controller 20,include a microcomputer and various kinds of circuits. The CPU of themicrocomputer realizes control of the command generator 21C and the PWMsignal generator 22C by reading out and executing a program stored inthe ROM. One or all of the command generator 21C and the PWM signalgenerator 22C may be configured by the hardware such as an ASIC, an FPGAor the like.

The power converter 10C is not limited to the examples shown in FIG. 10.FIG. 12 is a view showing a configuration example of another powerconverter 10C relating to the fourth embodiment. The power converter 10Cshown in FIG. 12 is configured as a multilevel output, and specifically,includes a gate drive circuit 11C′, a current detector 12A, and amultilevel inverter circuit(e.g., a three-phase three-level invertercircuit 13C′).

As shown in FIG. 12, the three-phase three-level inverter circuit 13C′includes switching elements Q21 to Q24, Q31 to Q34, and Q41 to Q44 andcapacitors C21 and C22. A protective rectifying element isparallel-connected to each of the switching elements Q21 to Q24, Q31 toQ34, and Q41 to Q44.

The gate drive circuit 11C′ generates gate signals PA1 to PA3, PB1 toPB3, and PC1 to PC3 based on the PWM signals PA, PB, and PC outputtedfrom the controller 20C. Description will now be made on an example inwhich the gate drive circuit 11C′ generates the gate signals PA1 to PA3using the PWM signal PA.

For example, if the PWM signal PA is of a high level, the gate drivecircuit 11C′ keeps the gate signal RA1 at a high level and keeps thegate signals PA2 and PA3 at a low level. Thus, the three-phasethree-level inverter circuit 13C′ outputs a power source voltage Vdcfrom the U-phase output terminal Tu.

For example, if the PWM signal PA is of a low level, the gate drivecircuit 11C′ keeps the gate signal PA3 at a high level and keeps thegate signals PA1 and PA2 at a low level. Thus, the three-phasethree-level inverter circuit 13C′ outputs a zero potential from theU-phase output terminal Tu.

For example, if the PWM signal PA is of a middle level, the gate drivecircuit 11C′ keeps the gate signal PA3 at a high level and keeps thegate signals PA1 and PA3 at a low level. Thus, the three-phasethree-level inverter circuit 13C′ outputs a voltage (Vdc/2) equal to onehalf of the power source voltage Vdc from the U-phase output terminalTu.

With respect to the PWM signals PB and PC, just like the PWM signal PA,the gate drive circuit 11C′ generates gate signals PB1 to PB3 and PC1 toPC3. In this way, the power converter 10C converts the DC voltageinputted from the power source 2 through the input terminals Tp and Tnto the three-phase AC voltage using the switching operations of theswitching elements Q21 to Q24, Q31 to Q34, and Q41 to Q44 and outputsthe three-phase AC voltage from the output terminals Tu, Tv, and Tw tothe load 3A through the output lines 6 a to 6 c.

[Fifth Embodiment]

Next, description will be made on a power converting apparatus relatingto a fifth embodiment. The power converting apparatus relating to thefifth embodiment differs from the power converting apparatus 1B relatingto the third embodiment in that the power converting apparatus of thefifth embodiment generates a PWM signal using a space vector method. Inthe following description, the constituent elements having the samefunctions as those of the power converting apparatus 1B will bedesignated by like reference symbols. No duplicate description will bemade thereon.

FIG. 13 is a view showing a configuration example of a power convertingapparatus 1D relating to the fifth embodiment. The power convertingapparatus 1D includes a power converter 10B and a controller 20D andoutputs a three-phase AC current to a load 3A. A configuration exampleof the controller 20D will now be described in detail.

As shown in FIG. 13, the controller 20D includes a command generator 21Dand a PWM signal generator 22D. The command generator 21D includes acurrent command generator 23A and a current controller 24D. The currentcontroller 24D generates a voltage command vector Vs* (one example of avoltage command) such that a deviation between the phase current commandIuvw* and the output phase current Iuvw becomes zero.

The PWM signal generator 22D includes a mode switcher 31D, a selector35, a calculator 36, a changer 37, and a generator 38.

The mode switcher 31D outputs a mode signal Sm to the calculator 36 andswitches a first mode and a second mode. For example, if a detectiontemperature Tc is loser than a predetermined value, the mode switcher31D outputs a mode signal Sm indicative of the first mode to thecalculator 36. If the detection temperature Tc is equal to or higherthan the predetermined value, the mode switcher 31D outputs a modesignal Sm indicative of the second mode to the calculator 36.

Based on the voltage command vector Vs*, for each (and preferably,every) updating cycle Ts, the selector 35 selects a combination of twoor more (e.g. two) zero voltage vectors and one or more (e.g., two)non-zero voltage vectors from a plurality of voltage vectors. FIG. 14 isan explanatory view of a space vector method. Θv is a phase anglebetween the voltage vector V1 and the voltage command vector Vs.

In FIG. 14, there are shown eight voltage vectors including zero voltagevectors V0 and V7 and non-zero voltage vectors V1 to V6. The selector 35selects, e.g., two non-zero voltage vectors V1 and V2 adjoining thevoltage command vector Vs* and zero voltage vectors V0 and V7.

In this case, every updating cycle Ts, the selector 35 alternatelyswitches, for example, a pattern (hereinafter referred to as “firstselection pattern”) in which the selector 35 selects the voltage vectorsin the order of V0, V1, V2 and V7 and a pattern (hereinafter referred toas “second selection pattern”) in which the selector 35 selects thevoltage vectors in the order of V7, V2, V1, and V0 which is the reverseorder of that of the first selection pattern.

In FIG. 14, V1 (100) indicates the state of a U-phase, a V-phase, and aW-phase attributable to the voltage vector V1 and indicates the state inwhich the switching element Q11 existing above the U-phase is turned onand in which the switching elements Q14 and Q16 existing below theV-phase and the W-phase are turned on.

The calculator 36 (which may also be called “an output periodcalculator”) calculates output periods of the voltage vectors selectedby the selector 35. For example, if the selector 35 selects the non-zerovoltage vectors V1 and V2, the calculator 36 calculates an output periodt1 of the non-zero voltage vector V1 and an output period t2 of thenon-zero voltage vector V2 using, e.g., the following formulae (1) and(2):

$\begin{matrix}{t_{1} = {\frac{2}{\sqrt{3}}\frac{V_{s}^{*}}{V_{\max}}{T_{s} \cdot {\sin\left( {\frac{\pi}{3} - \theta_{V}} \right)}}}} & (1) \\{t_{2} = {\frac{2}{\sqrt{3}}\frac{V_{s}^{*}}{V_{\max}}{T_{s} \cdot \sin}\mspace{14mu}\theta_{V}}} & (2)\end{matrix}$

where V_(max) denotes a maximum value of the voltage command.

The calculator 36 calculates an output period t0 of the zero voltagevector V0 and an output period t7 of the zero voltage vector V7 bydividing a period (=Ts−t1−t2) obtained by subtracting the total sum ofthe output periods t1 and t2 of the non-zero voltage vectors V1 and V2from the updating cycle Ts, into two periods.

The calculator 36 outputs the information on the output periods of thevoltage vectors to the changer 37 in the order of the voltage vectorsselected by the selector 35. For example, if the voltage vectors V0, V1,V2, and V7 are selected by the selector 35 in the first selectionpattern, the calculator 36 outputs the information on the output periodsin the order of the output periods t0, t1, t2, and t7.

If the mode signal Sm indicative of the first mode is outputted from themode switcher 31D, the changer 37 outputs the information on the outputperiods acquired from the calculator 36 as it is. For example, uponacquiring the information on the output periods t0, t1, t2, and t7 fromthe calculator 36, the changer 37 outputs the information on the outputperiods t0, t1, t2, and t7 as it is.

On the other hand, if the mode signal Sm indicative of the second modeis outputted from the mode switcher 31D, the changer 37 changes theoutput periods of two or more zero voltage vectors such that the outputperiods of two or more zero voltage vectors among the output periodscalculated by the calculator 36 are replaced by the output period of onezero voltage vector corresponding to the total sum output period of twoor more zero voltage vectors.

For example, upon acquiring the information on the output periods t0,t1, t2, and t7 from the calculator 36, the changer 37 adds up the outputperiods t0 and t7. The addition result is used as the output period ofone of the output periods t0 and t7. The other of the output periods t0and t7 is made 0. Thus, two zero voltage vectors to be outputted ischanged to one.

If the addition result is used as the output period t0, the changer 37outputs the information on the output periods t0, t1, and t2. If theaddition result is used as the output period t7, the changer 37 outputsthe information on the output periods t1, t2, and t7.

Based on the information on the output periods outputted from thechanger 37, the generator 38 (which may also be called “a generatingcircuit”) generates PWM signals PA, PB, and PC. Specifically, thegenerator relating to the present embodiment generates the PWM signalswhich are set such that the output period of one zero voltage vectoroutputted from the changer is used as one first period and such that theoutput periods of one or more non-zero voltage vectors outputted fromthe changer are used as one or more second periods. The generator 38outputs the generated PWM signals PA, PB, and PC to the power converter10B (the gate drive circuit 11B).

For example, upon sequentially acquiring, as the information on theoutput periods in the first mode, the information on the output periodst0, t1, t2, and t7 and the information on the output periods t7, t2, t1,and t0 from the changer 37, the generator 38 generates PWM signals PA,PB, and PC as shown in FIG. 15A. FIG. 15A is a view showing therelationship between the voltage vectors, the output periods and the PWMsignals in the first mode.

Furthermore, upon acquiring, as the information on the output periods inthe second mode, the information on the output periods t0, t1, and t2and the information on the output periods t2, t1, and t0 from thechanger 37, the generator 38 generates PWM signals PA, PB, and PC asshown in FIG. 15B. FIG. 15B is a view showing the relationship betweenthe voltage vectors, the output periods and the PWM signals in thesecond mode.

Moreover, upon acquiring, as the information on the output periods inthe second mode, the information on the output periods t1, t2, and t7and the information on the output periods t7, t2, and t1 from thechanger 37, the generator 38 generates PWM signals PA, PB, and PC asshown in FIG. 15C. FIG. 15C is a view showing the relationship betweenthe voltage vectors, the output periods and the PWM signals in thesecond mode.

As shown in FIGS. 15B and 15C, in case of the second mode, the generator38 generates PWM signals PA, PB, and PC in which the output period t0 orthe output period t7 changed by the changer 37 is used as a first periodT1 and in which the output periods of one or more non-zero voltagevectors (e.g., the output periods of t1 and t2 of the two non-zerovoltage vectors V1 and V2 as shown in FIGS. 15B and 15C) are used as twosecond periods T2.

Thus, in the second mode, just like the power converting apparatus 1B,the power converting apparatus 1D can make the number of turn-on timesof a PWM pulse, namely the number of switching times, equal to twothirds of that available in the first mode. This makes it possible toreduce a switching loss while suppressing an increase in the dead time.

In the foregoing description, the order of the voltage command vectorsVs* is set by the selector 35.

Alternatively, the order of the voltage command vectors may be set bythe generator 38. In this case, the generator 38 sets the order of thevoltage vectors based on, e.g., the voltage command vectors Vs* and themode signals Sm.

Now, description will be made on one example of the flow of theprocessing in the controller 20D. FIG. 16 is a flowchart showing oneexample of the flow of the processing in the controller 20D.

As shown in FIG. 16, the command generator 21D of the controller 20Dgenerates a voltage command vector Vs* (step S21). Then, the PWM signalgenerator 22D of the controller 20D determines whether now is theupdating timing TR of the voltage command vector Vs* (step S22).

If it is determined that now is the updating timing TR of the voltagecommand vector Vs* (if Yes at step S22), the PWM signal generator 22Dselects a voltage vector based on the voltage command vector Vs* (stepS23). The PWM signal generator 22D calculates the output period of theselected voltage vector (step S24).

Next, the PWM signal generator 22D determined whether it is the secondmode (step S25). For example, if the voltage command V* is smaller thana predetermined value, the PWM signal generator 22D determines that itis the second mode.

If it is determined that it is the second mode (if Yes at step S25), thePWM signal generator 22D makes the output periods of multiple zerovoltage vectors become the output period of one zero voltage vector(step S26).

If the processing of step S26 is completed or if it is determined atstep S25 that it is not the second mode (if No at step S25), the PWMsignal generator 22D generates PWM signals PA, PB, and PC based on theoutput period of voltage vectors (step S27).

Meanwhile, the method for controlling the power converting apparatusrelating to the present embodiment may include, e.g., a commandgenerating process and a signal generating process, and correspond tothe processing follow of the controller 20D and/or its elements asdescribed above. Specifically, the processing flow at step S21 may be anexample or an element of the command generating process, and theprocessing follow at steps S22 to S27 may be an example or an element ofthe signal generating process. Further, the controller 20D may also bean example of the control device relating to the present embodiment,and, similarly to the controller 20, include a microcomputer and variouskinds of circuits. The CPU of the microcomputer realizes control of thecommand generator 21D and the PWM signal generator 22D by reading outand executing a program stored in the ROM. One or all of the commandgenerator 21D and the PWM signal generator 22D may be configured by thehardware such as an ASIC, an FPGA or the like.

[Sixth Embodiment]

Next, description will be made on a power converting apparatus relatingto a sixth embodiment. The power converting apparatus relating to thesixth embodiment differs from the power converting apparatus 1C relatingto the fourth embodiment in that the power converting apparatus of thesixth embodiment generates a PWM signal using a space vector method. Inthe following description, the constituent elements having the samefunctions as those of the power converting apparatuses 1C and 1D will bedesignated by like reference symbols. No duplicate description will bemade thereon.

FIG. 17 is a view showing a configuration example of a power convertingapparatus 1E relating to the sixth embodiment. The power convertingapparatus 1E includes a power converter 10C and a controller 20E andoutputs a three-phase AC current to a load 3A. A configuration exampleof the controller 20E will now be described in detail.

As shown in FIG. 17, the controller 20E includes a command generator 21Dand a PWM signal generator 22E. The PWM signal generator 22E includes amode switcher 31D, a selector 35E, a calculator 36E, a changer 37E, anda generator 38E.

Based on a voltage command vector Vs* (one example of a voltagecommand), every updating cycle Ts, the selector 35E selects acombination of three zero voltage vectors and four non-zero voltagevectors from twenty seven kinds of voltage vectors. FIG. 18 is anexplanatory view of a space vector method.

In FIG. 18, there are shown three zero voltage vectors Op, Om, and On,and twenty four non-zero voltage vectors a(1)˜a(3), b(1)˜b(3),ap(1)˜ap(3), an(1)˜an(3), bp(1)˜bp(3), bn(1)˜bn(3), and z(1)˜z(6).

FIG. 19 is a view showing a correspondence example of a voltage commandvector Vs* and space vectors. If the voltage command vector Vs* is inthe state shown in FIG. 19, the selector 35E selects, for example, fournon-zero voltage vectors ap, an, bp, and bn, and three zero voltagevectors Op, Oo, and On which adjoin the voltage command vector Vs*.

In this case, every updating cycle Ts, the selector 35E alternatelyswitches, for example, a pattern (hereinafter referred to as “firstselection pattern”) in which the selector 35E selects the voltagevectors in the order of On→an→bn→Oo→ap→bp→Op and a pattern (hereinafterreferred to as “second selection pattern”) in which the selector 35Eselects the voltage vectors in the order of Op→bp→ap→Oo→bn→an→On whichis the reverse order of that of the first selection pattern. In FIG. 19,for example, PPO indicates the output states of the U-phase, V-phase,and W-phase of the voltage vector ap(1) and shows a state in which theswitching elements Q21, Q22, Q31, and Q32 existing above the U-phase andthe V-phase are turned on and in which the switching elements Q42 andQ43 existing at the center of the W-phase are turned on.

The calculator 36E (which may also be called “an output periodcalculator”) outputs the information on the output periods of thevoltage vectors to the changer 37E in the order of the voltage vectorsselected by the selector 35E. For example, if the voltage vectors of thefirst selection pattern are selected by the selector 35E, the calculator36E outputs the information on the output periods in the order of thevoltage vectors On, an, bn, Oo, ap, bp, and Op.

The calculator 36E calculates output periods of the voltage vectorsselected by the selector 35E. For example, if the non-zero voltagevectors ap, an, bp, and bn are selected by the selector 35E, thecalculator 36E finds output periods tap, tan, tbp, and tbn of therespective non-zero voltage vectors ap, an, bp, and bn.

Furthermore, the calculator 36E finds output periods top, too, and tonof the zero voltage vectors Op, Oo, and On by dividing a period(=Ts−tap−tan−tbp−tbn) obtained by subtracting the total sum of theoutput periods tap, tan, tbp, and tbn from the updating cycle Ts, intothree periods.

If the mode signal Sm indicative of the first mode is outputted from themode switcher 31D, the changer 37E outputs the information on the outputperiods acquired from the command updater 32C as it is. For example,upon acquiring the information on the output periods ton, tan, tbn, too,tap, tbp, and top from the calculator 36E, the changer 37E outputs theinformation on the output periods ton, tan, tbn, too, tap, tbp, and topas it is.

On the other hand, if the mode signal Sm indicative of the second modeis outputted from the mode switcher 31D, the changer 37E changes theoutput periods of three or more zero voltage vectors such that theoutput periods of three or more zero voltage vectors among the outputperiods calculated by the calculator 36E are replaced by the outputperiod of one zero voltage vector corresponding to the total sum outputperiod of three or more zero voltage vectors.

For example, upon acquiring the information on the output periods ton,tan, tbn, too, tap, tbp, top from the calculator 36E, the changer 37Eadds up the output periods ton, too, and top. The addition result isused as a new output period too. The output periods ton and top are made0. Thus, three zero voltage vectors to be outputted is changed to one.

If the addition result is used as a new output period too as mentionedabove, the changer 37E sequentially outputs the information on theoutput periods tan, tbn, too, tap, and tbp.

Based on the information on the output periods outputted from thechanger 37E, the generator 38E (which may also be called “a generatingcircuit”) generates PWM signals PA, PB, and PC. The generator 38Eoutputs the generated PWM signals PA, PB, and PC to the power converter10C (the gate drive circuit 11C).

For example, upon sequentially acquiring, as the information on theoutput periods in the first mode, the information on the output periodston, tan, tbn, too, tap, tbp, and top and the information on the outputperiods top, tbp, tap, too, tbn, tan, and ton from the changer 37E, thegenerator 38E generates PWM signals PA, PB, and PC as shown in FIG. 20A.FIG. 20A is a view showing the relationship between the voltage vectors,the output periods and the PWM signals in the first mode.

Furthermore, upon acquiring, as the information on the output periods inthe second mode, the information on the output periods tan, tbn, too,tap, and tbp and the information on the output periods tbp, tap, too,tbn, and tan from the changer 37E, the generator 38E generates PWMsignals PA, PB, and PC as shown in FIG. 20B. FIG. 20B is a view showingthe relationship between the voltage vectors, the output periods and thePWM signals in the second mode.

As shown in FIG. 20B, in case of the second mode, the generator 38Egenerates PWM signals PA, PB, and PC in which the output period toochanged by the changer 37E is used as a first period T1 and in whichfour output periods tan, tbn, tap, and tbp of non-zero voltage vectorsan, bn, ap, and by are used as four second periods T2.

Thus, in the second mode, just like the power converting apparatus 1C,the power converting apparatus 1E can make the number of turn-on timesof a PWM pulse, namely the number of switching times, equal to twothirds of that available in the first mode. This makes it possible toreduce a switching loss while suppressing an increase in the dead time.

In the foregoing description, the order of the voltage command vectorsVs* is set by the selector 35E. Alternatively, the order of the voltagecommand vectors may be set by the generator 38E. In this case, thegenerator 38E sets the order of the voltage vectors based on, e.g., thevoltage command vectors Vs* and the mode signals Sm.

The flow of the processing in the controller 20E is the same as the flowof the processing shown in FIG. 16. Therefore, no further detaileddescription will be made thereon.

Meanwhile, the method for controlling the power converting apparatusrelating to the present embodiment may include, e.g., a commandgenerating process and a signal generating process, and correspond tothe processing follow of the controller 20E and/or its elements asdescribed above.

Specifically, the processing flow at step S21 as shown in FIG. 16 may bean example or an element of the command generating process, and theprocessing follow at steps S22 to S27 as shown in FIG. 16 may be anexample or an element of the signal generating process.

Further, the controller 20E may also be an example of the control devicerelating to the present embodiment, and, similarly to the controller 20,include a microcomputer and various kinds of circuits. The CPU of themicrocomputer realizes control of the command generator 21D and the PWMsignal generator 22E by reading out and executing a program stored inthe ROM. One or all of the command generator 21D and the PWM signalgenerator 22E may be configured by the hardware such as an ASIC, an FPGAor the like.

[Seventh Embodiment]

Next, description will be made on a power converting apparatus relatingto a seventh embodiment. The power converting apparatus relating to theseventh embodiment differs from the power converting apparatus 1Brelating to the third embodiment in that the power converting apparatusof the seventh embodiment generates a PWM signal in which one firstperiod T1 is set by executing a state inversion process with respect tothe PWM signal outputted from a PWM signal generator. In the followingdescription, the constituent elements having the same functions as thoseof the power converting apparatus 1B will be designated by likereference symbols. No duplicate description will be made thereon.

FIG. 21 is a view showing a configuration example of a power convertingapparatus 1F relating to the seventh embodiment. The power convertingapparatus 1F includes a power converter 10B and a controller 20F andoutputs a three-phase AC current to a load 3A. A configuration exampleof the controller 20F will now be described in detail.

As shown in FIG. 21, the controller 20F includes a command generator21A, a PWM signal generator 22F, a mode switcher 26, and a stateinverter 27.

The PWM signal generator 22F includes a carrier signal generator 30B, acommand updater 32B, a comparator 34B, and an inversion time calculator39. Using the carrier signal generator 30B, the command updater 32B, andthe comparator 34B, the PWM signal generator 22F generates PWM signalsPA, PB, and PC which are the same as those generated when the PWM signalgenerator 22B relating to the third embodiment is operated in the firstmode.

The inversion time calculator 39 determines the states of the PWMsignals PA, PB, and PC, based on the peak value Vp and the bottom valueVb of the carrier signal Vc and the phase voltage commands Vu*, Vv*, andVw*, and calculates the inversion time of each of the PWM signals PA,PB, and PC.

FIG. 22 is an explanatory view of a method for calculating the inversiontime of each of the PWM signals PA, PB, and PC. Description will now bemade on a period Ts1 in which the carrier signal Vc migrates from thepeak to the bottom and a period Ts2 in which the carrier signal Vcmigrates from the bottom to the peak.

In the period Ts1, the inversion time calculator 39 calculates adifference ΔVu1 between the peak value Vp of the carrier signal Vc andthe phase voltage command Vu* and calculates an output period t0 of azero voltage (NNN) based on the difference ΔVu1. Furthermore, theinversion time calculator 39 calculates a difference ΔVv1 between thephase voltage command Vu* and the phase voltage command Vv* andcalculates an output period t1 of a non-zero voltage (PNN) based on thedifference ΔVv1.

Moreover, the inversion time calculator 39 calculates a difference ΔVw1between the phase voltage command Vv* and the phase voltage command Vw*and calculates an output period t2 of a non-zero voltage (PPN) based onthe difference ΔVw1. In addition, the inversion time calculator 39calculates an output period t7 of a zero voltage vector (PPP) from theoutput periods t0, t1, and t2.

The inversion time calculator 39 sets, using the output period t7, aninverting time RA for the PWM signal PA to range from the time t11 tothe time t12 and sets, using the output period t1, an inverting time RBfor the PWM signal PB to range from the time t12 to the time t13.

Furthermore, the inversion time calculator 39 sets, using the outputperiod t2, an inverting time RC for the PWM signal PC to range from thetime t13 to the time t14.

In the period Ts2, the inversion time calculator 39 calculates adifference ΔVw2 between the bottom value Vb of the carrier signal Vc andthe phase voltage command Vw* and calculates an output period t7 of azero voltage (PPP) based on the difference ΔVw2. Furthermore, theinversion time calculator 39 calculates a difference ΔVv2 between thephase voltage command Vw* and the phase voltage command Vv* andcalculates an output period t2 of a non-zero voltage (PPN) based on thedifference ΔVv2.

Moreover, the inversion time calculator 39 calculates a difference ΔVu2between the phase voltage command Vv* and the phase voltage command Vu*and calculates an output period t1 of a non-zero voltage (PNN) based onthe difference ΔVu2. In addition, the inversion time calculator 39calculates an output period t0 of a zero voltage vector (NNN) from theoutput periods t7, t2, and t1.

The inversion time calculator 39 sets, using the output period t7, aninverting time RA for the PWM signal PA to range from the time t16 tothe time t17 and sets, using the output period t2, an inverting time RBfor the PWM signal PB to range from the time t15 to the time t16.Furthermore, the inversion time calculator 39 sets, using the outputperiod t1, an inverting time RC for the PWM signal PC to range from thetime t14 to the time t15.

Just like the mode switcher 31B, the mode switcher 26 switches the firstmode and the second mode depending on the mode signal Sm. For example,if the detection temperature Tc is lower than a predetermined value, themode switcher 26 outputs the mode signal Sm indicative of the first modeto the state inverter 27. If the detection temperature Tc is equal to orhigher than the predetermined value, the mode switcher 26 outputs themode signal Sm indicative of the second mode to the state inverter 27.

If the mode signal Sm indicative of the first mode is outputted from themode switcher 26, the state inverter 27 directly outputs the PWM signalsPA, PB, and PC inputted from the PWM signal generator 22F, as PWMsignals PA′, PB′, and PC′.

Thus, every updating cycle Ts of the phase voltage command Vuvw*, thecontroller 20F can repeatedly outputs the PWM signals PA′, PB′, and PC′having a control pattern in which the PWM signals PA′, PB′, and PC′migrate in the order of a first period T1, a second period T2, and afirst period T1.

On the other hand, if the mode signal Sm indicative of the second modeis outputted from the mode switcher 26, the state inverter 27 inverts apart of each of the PWM signals PA, PB, and PC based on the invertingtime RA, RB, and RC and generates and outputs PWM signals PA′, PB′, andPC′.

For example, the state inverter 27 inverts the PWM signal PA andgenerates PWM signal PA′ during the period between the time t11 and thetime t12 and during the period between the time t16 and the time t17.Furthermore, the state inverter 27 inverts the PWM signal PB andgenerates PWM signal PB′ during the period between the time t12 and thetime t13 and during the period between the time t15 and the time t16.Moreover, the state inverter 27 inverts the PWM signal PC and generatesPWM signal PC′ during the period between the time t13 and the time t14and during the period between the time t14 and the time t15.

Thus, in the second mode, every updating cycle Ts, the controller 20Falternately outputs a PWM signal having a control pattern in which thePWM signal migrates in the order of a first period T1 and a secondperiod T2 during one updating cycle Ts and a PWM signal having a controlpattern in which the PWM signal migrates in the order of a second periodT2 and a first period T1 during one updating cycle Ts.

Accordingly, in the second mode, just like the power convertingapparatus 1B, the power converting apparatus 1F can make the number ofturn-on times of a PWM pulse, namely the number of switching times,equal to two thirds of that available in the first mode. This makes itpossible to reduce a switching loss while suppressing an increase in thedead time.

Just like the power converting apparatus 1F, the power convertingapparatus 1C relating to the fourth embodiment may be provided with aninversion time calculator and a state inverter in place of the shifter33C. In this case, the inversion time calculator calculates the outputperiods of the respective zero voltages On, Oo, and Op and the outputperiods of the respective non-zero voltages an, bn, ap, and by from thepeak value Vp and the bottom value Vb of the carrier signal Vc and thephase voltage command Vuvwpn* and calculates the inverting time RA, RB,and RC from these output periods. The state inverter inverts a part ofeach of the PWM signals PA, PB, and PC, based on the inverting time RA,RB, and RC, and generates and outputs PWM signals PA′, PB′, and PC′.

Just like the power converting apparatus 1F, the power convertingapparatus 1 relating to the first embodiment may be provided with aninversion time calculator and a state inverter in place of the shifter33. In this case, the inversion time calculator calculates the outputperiods of the respective zero voltages and the output periods of therespective non-zero voltages from the peak value Vp of the carriersignal Vc1 or the bottom value Vb of the carrier signal Vc2 and thevoltage command V* and calculates the inverting time from these outputperiods. The state inverter inverts a part of each of the PWM signalsL1, L2, R1, and R2 based on the inverting time.

Further, the controller 20F may also be an example of the control devicerelating to the present embodiment, and, similarly to the controller 20,include a microcomputer and various kinds of circuits. The CPU of themicrocomputer realizes control of the command generator 21A, the PWMsignal generator 22F, the mode switcher 26, and the state inverter 27 byreading out and executing a program stored in the ROM. One or all of thecommand generator 21A, the PWM signal generator 22F, the mode switcher26, and the state inverter 27 may be configured by the hardware such asan ASIC, an FPGA or the like.

[Eighth Embodiment]

Next, description will be made on a power converting apparatus relatingto an eighth embodiment. The power converting apparatus relating to theeighth embodiment differs from the power converting apparatus 1Frelating to the seventh embodiment in that the power convertingapparatus of the eighth embodiment generates a PWM signal in which onefirst period T1 is set by executing a state inversion process withrespect to the output of a gate drive circuit. In the followingdescription, the constituent elements having the same functions as thoseof the power converting apparatus 1F will be designated by likereference symbols. No duplicate description will be made thereon.

FIG. 23 is a view showing a configuration example of a power convertingapparatus 1G relating to the eighth embodiment. As shown in FIG. 23, thepower converting apparatus 1G may include a power converter 10G and acontroller 20G and output a three-phase AC current to a load 3A. Thepower converter 10G may include a three-phase two-level inverter circuit13B and a current detector 12A.

The controller 20G may include a command generator 21A, a PWM signalgenerator 22F, a mode switcher 26, a gate drive circuit 11B, and a stateinverter 27G. The PWM signal generator 22F generates PWM signals PA, PB,and PC which are the same as those generated when the PWM signalgenerator 22B is operated in the first mode.

If the mode signal Sm indicative of the first mode is outputted from themode switcher 26, the state inverter 27G directly outputs the gatesignals S1 to S6 inputted from the gate drive circuit 11B, as gatesignals S1′ to S6′. The gate signals S1 to S6 and S1′ to S6′ are PWMsignals but will be referred to as “gate signals” in order todistinguish them from the PWM signals PA, PB, and PC.

Thus, every updating cycle Ts of the phase voltage command Vuvw*, thecontroller 20G can repeatedly output a PWM signal having a controlpattern in which the PWM signal migrates in the order of a first periodT1, a second period T2 and a first period T1.

On the other hand, if the mode signal Sm indicative of the second modeis outputted from the mode switcher 26, the state inverter 27G inverts apart of each of the gate signals S1 to S6 inputted from the gate drivecircuit 11B, based on the inverting time RA, RB, and RC and the carriersignal Vc, and generates and outputs gate signals S1′ to S6′.

For example, the state inverter 27G inverts the gate signals S1 and S2and generates gate signals S1′ and S2′ during the period between thetime t11 and the time t12 and during the period between the time t16 andthe time t17. Furthermore, the state inverter 27G inverts the gatesignals S3 and S4 and generates gate signals S3′ and S4′ during theperiod between the time t12 and the time t13 and during the periodbetween the time t15 and the time t16. Moreover, the state inverter 27Ginverts the gate signals S5 and S6 and generates gate signals S5′ andS6′ during the period between the time t13 and the time t14 and duringthe period between the time t14 and the time t15.

Thus, in the second mode, every updating cycle Ts, the controller 20Galternately outputs a PWM signal having a control pattern in which thePWM signal migrates in the order of a first period T1 and a secondperiod T2 during one updating cycle Ts and a PWM signal having a controlpattern in which the PWM signal migrates in the order of a second periodT2 and a first period T1 during one updating cycle Ts.

Accordingly, in the second mode, just like the power convertingapparatus 1B, the power converting apparatus 1G can make the number ofturn-on times of a PWM pulse, namely the number of switching times,equal to two thirds of that available in the first mode. This makes itpossible to reduce a switching loss while suppressing an increase in thedead time.

While description has been made on an example in which the controllers20, 20B to 20G, and 17 relating to the aforementioned embodiments do notchange the updating cycle Ts, it may be possible to, in addition to themode switching, change the updating cycle Ts depending on, e.g., thefrequency of the output voltage (or the voltage command).

While the controllers 20, 20B to 20G, and 17 relating to theaforementioned embodiments are configured to change the modes based onthe temperature of the power converting apparatuses 1 and 1A to 1G, itmay be possible to change the modes based on, e.g., the frequency of theoutput voltage (or the voltage command) and the distortion of the outputvoltage.

For example, the mode switchers 26, 31, and 31B of the power convertingapparatuses 1 and 1A to 1G select the first mode if the frequency of theoutput voltage (or the voltage command) is equal to or higher than apredetermined value and select the second mode if the frequency of theoutput voltage (or the voltage command) is lower than the predeterminedvalue.

As another example, the power converting apparatuses 1 and 1A to 1G mayinclude a distortion detector for detecting distortion of an outputvoltage. In this case, the mode switchers 26, 31, and 31B select thefirst mode if the distortion of the output voltage detected by thedistortion detector is smaller than a predetermined value and select thesecond mode if the distortion of the output voltage is equal to orlarger than the predetermined value.

The command generators 21, 21A, 21C, and 21D may generate the voltagecommand V* or Vuvw* using, e.g., the voltage command of a dq−axiscomponent of rectangular coordinates which rotate in synchronism withthe phase of the output voltage of the power converter 10, 10A, 10B, 100or 10G or the phase (electric angle) of the load 3 or 3A.

In the second mode, the PWM signal generators 22 and 22B to 22G may useone of the bottom and the peak of the carrier signal as the updatingtiming of the voltage command.

In the description made above, the power converting apparatus 1 relatingto the first embodiment generates the

PWM signal for the single-phase inverter circuit 13 using the carriercomparison method. Alternatively, just like the power convertingapparatuses 1C and 1D relating to the fourth and fifth embodiments, thepower converting apparatus 1 may generate the PWM signal for thesingle-phase inverter circuit 13 using the space vector method. In theaforementioned embodiments, description has been made on the PWM signalfor the inverter circuit of three levels or less. Even in case of thePWM signal for the inverter circuit of more than three levels, it ispossible to reduce a switching loss while suppressing an increase in thedead time, by outputting a PWM signal in which one first period T1 andone or more second periods T2 are combined with each other.

Other new effects, modifications, combinations, sub-combinations, andalterations can be readily derived by those skilled in the relevant art.For that reason, the broad aspect of the present disclosure is notlimited to the specific details and the representative embodiments shownand described above. Accordingly, the present disclosure can be modifiedin many different forms depending on design requirements and otherfactors without departing from the spirit and scope thereof defined bythe appended claims and the equivalents thereof.

What is claimed is:
 1. A power converting apparatus for convertingelectric power between a power source and a load, comprising: a powerconverter configured to output a voltage to the load; and a controllerconfigured to output a PWM signal which is generated in response to avoltage command to the power converter, wherein the power converterincludes a plurality of switching elements driven based on the PWMsignal, wherein the controller is configured to generate the PWM signalsuch that a first period during which a zero voltage is outputted and asecond period during which a non-zero voltage is outputted are adjustedaccording to the voltage command, wherein the controller is configuredto output the PWM signal which is set such that one first period and oneor more second periods exist within an updating cycle of the voltagecommand, to the power converter, wherein the controller includes a PWMsignal generator configured to generate the PWM signal, wherein the PWMsignal generator is configured to alternately output the PWM signalhaving a first pattern and the PWM signal having a second pattern to thepower converter, the first pattern being set such that one first periodand one second period exist within the updating cycle and the firstperiod is followed by the second period, the second pattern being setsuch that one first period and one second period exist within theupdating cycle and the second period is followed by the first period,wherein the controller is configured to be operable in any one of afirst mode and a second mode, wherein, in the first mode, the PWM signalgenerator repeatedly outputs, for each updating cycle of the voltagecommand, a PWM signal having a third pattern to the power converter, thethird pattern being set to sequentially cause the first period, thesecond period, and the first period in that order, wherein, in thesecond mode, the PWM signal generator alternately outputs, for eachupdating cycle of the voltage command, the PWM signal having the firstpattern and the PWM signal having the second pattern to the powerconverter, and wherein the PWM signal generator includes a mode switcherconfigured to switch the first mode and the second mode based on apreset condition.
 2. The power converting apparatus of claim 1, whereinthe voltage outputted by the power converter is an AC voltage having oneor more phases, wherein the voltage command includes one or more phasevoltage commands respectively corresponding to said one or more phases,wherein the controller further includes a command generator configuredto generate said one or more phase voltage commands, and wherein the PWMsignal generator includes: a carrier signal generator configured togenerate a carrier signal; a comparator configured to compare each phasevoltage command with the carrier signal and generate the PWM signal foreach phase based on a result thereof; and a shifter configured torelatively shift said phase voltage commands which are to be compared bythe comparator, said phase voltage commends being relatively shiftedwith respect to the carrier signal generated from the carrier signalgenerator, based on one of a peak value and a bottom value of thecarrier signal.
 3. The power converting apparatus of claim 2, whereinthe shifter is configured to shift the phase voltage commands such thata greatest one among the phase voltage commands becomes a peak value ofthe carrier signal or such that a smallest one among the phase voltagecommands becomes a bottom value of the carrier signal.
 4. The powerconverting apparatus of claim 3, wherein the power converter isconfigured as a multilevel output, wherein the command generator isconfigured to generate a first phase voltage command and a second phasevoltage command with respect to each of the phases and output aplurality of first phase voltage commands and a plurality of secondphase voltage commands, and wherein the shifter is configured to shiftthe first phase voltage commands such that the greatest one among thefirst phase voltage commands becomes the peak value of the carriersignal and shift the second phase voltage commands such that thesmallest one among the second phase voltage commands becomes the bottomvalue of the carrier signal.
 5. The power converting apparatus of claim1, wherein the power converter is configured to selectively output oneof the zero voltage and the non-zero voltage to the load.
 6. The powerconverting apparatus of claim 1, wherein the controller is configured tooutput, for each updating cycle of the voltage command, the PWM signalwhich causes said one first period and said one or more second periodswhich are combined within said each updating cycle of the voltagecommand.
 7. A control device for controlling the power converter ofclaim 1, comprising: a command generator configured to generate thevoltage command; and a signal generator configured to generate the PWMsignal such that the first period and the second period are adjustedaccording to the voltage command, and output the PWM signal to the powerconverter, wherein the signal generator is configured to output the PWMsignal which is set such that one first period and one or more secondperiods exist within the updating cycle of the voltage command, to thepower converter.
 8. A method for controlling the power convertingapparatus of claim 1, comprising: a command generating process forgenerating the voltage command; and a signal generating process forgenerating the PWM signal such that the first period and the secondperiod during which a non-zero voltage is outputted are adjustedaccording to the voltage command, and outputting the PWM signal to thepower converter, wherein the signal generating process includesoutputting the PWM signal which is set such that one first period andone or more second periods exist within the updating cycle of thevoltage command.
 9. The method of claim 8, wherein, in the signalgenerating process, the PWM signal is outputted for each updating cycleof the voltage command, the PWM signal being set to cause said one firstperiod and said one or more second periods which are combined withinsaid each updating cycle.
 10. The power converting apparatus of claim 1,wherein the controller further includes a command generator configuredto generate the voltage command, and wherein the PWM signal generatorincludes: a selector configured to select a combination of two or morezero voltage vectors and one or more non-zero voltage vectors from aplurality of voltage vectors based on the voltage command; a calculatorconfigured to calculate output periods of the voltage vectors selectedby the selector; a changer configured to change the output periods ofsaid two or more zero voltage vectors such that the output periods ofsaid two or more zero voltage vectors among the output periodscalculated by the calculator are replaced by an output period of onezero voltage vector corresponding to a total sum of the output periodsof said two or more zero voltage vectors; and a generator configured togenerate the PWM signal which is set such that the output period of saidone zero voltage vector is used as said one first period and the outputperiods of said one or more non-zero voltage vectors are used as saidone or more second periods.
 11. A power converting apparatus forconverting electric power between a power source and a load, comprising:a power converter configured to output a voltage to the load; and acontroller configured to output a PWM signal which is generated inresponse to a voltage command to the power converter, wherein the powerconverter includes a plurality of switching elements driven based on thePWM signal, wherein the controller is configured to generate the PWMsignal such that a first period during which a zero voltage is outputtedand a second period during which a non-zero voltage is outputted areadjusted according to the voltage command, wherein the controller isconfigured to output the PWM signal which is set such that one firstperiod and one or more second periods exist within an updating cycle ofthe voltage command, to the power converter, wherein the controllerincludes a PWM signal generator configured to generate the PWM signal,wherein the PWM signal generator is configured to repeatedly output, foreach updating cycle of the voltage command, the PWM signal having apattern to the power converter, the pattern being set to sequentiallycause the second period, the first period and the second period in thatorder, wherein the controller is configured to be operable in any one ofa first mode and a second mode, wherein, in the first mode, the PWMsignal generator repeatedly outputs, for each updating cycle of thevoltage command, a PWM signal having a pattern which is set tosequentially cause the first period, the second period, the firstperiod, the second period, and the first period in that order, to thepower converter, wherein, in the second mode, the PWM signal generatorrepeatedly outputs, for each updating cycle of the voltage command, thePWM signal having the pattern which is set to sequentially cause thesecond period, the first period, and the second period in that order, tothe power converter, and wherein the PWM signal generator includes amode switcher configured to switch the first mode and the second modebased on a preset condition.
 12. The power converting apparatus of claim11, wherein the controller further includes a command generatorconfigured to generate the voltage command, and wherein the PWM signalgenerator includes: a selector configured to select a combination of twoor more zero voltage vectors and one or more non-zero voltage vectorsfrom a plurality of voltage vectors based on the voltage command; acalculator configured to calculate output periods of the voltage vectorsselected by the selector; a changer configured to change the outputperiods of said two or more zero voltage vectors such that the outputperiods of said two or more zero voltage vectors among the outputperiods calculated by the calculator are replaced by an output period ofone zero voltage vector corresponding to a total sum of the outputperiods of said two or more zero voltage vectors; and a generatorconfigured to generate the PWM signal which is set such that the outputperiod of said one zero voltage vector is used as said one first periodand the output periods of said one or more non-zero voltage vectors areused as said one or more second periods.
 13. The power convertingapparatus of claim 11, wherein the power converter is configured toselectively output one of the zero voltage and the non-zero voltage tothe load.
 14. The power converting apparatus of claim 11, wherein thecontroller is configured to output, for each updating cycle of thevoltage command, the PWM signal which causes said one first period andsaid one or more second periods which are combined within said eachupdating cycle of the voltage command.
 15. A control device forcontrolling the power converter of claim 11, comprising: a commandgenerator configured to generate the voltage command; and a signalgenerator configured to generate the PWM signal such that the firstperiod and the second period are adjusted according to the voltagecommand, and output the PWM signal to the power converter, wherein thesignal generator is configured to output the PWM signal which is setsuch that one first period and one or more second periods exist withinthe updating cycle of the voltage command, to the power converter.
 16. Amethod for controlling the power converting apparatus of claim 11,comprising: a command generating process for generating the voltagecommand; and a signal generating process for generating the PWM signalsuch that the first period and the second period are adjusted accordingto the voltage command, and outputting the PWM signal to the powerconverter, wherein the signal generating process includes outputting thePWM signal which is set such that one first period and one or moresecond periods exist within the updating cycle of the voltage command.17. The method of claim 16, wherein, in the signal generating process,the PWM signal is outputted for each updating cycle of the voltagecommand, the PWM signal being set to cause said one first period andsaid one or more second periods which are combined within said eachupdating cycle.